Thunderbird says is not an imap4 mail server Print

Problem Description

Thunderbird's activity log says Mail server xyz is not an IMAP4 Server

Problem Resolution

As described in this MozillaZine KB article, other than some kind of actual connectivity problem like a firewall or routing issue, the capabilities list is where you want to look closely. You'll know it's the capabilities list if the issue is persistent across time and multiple connections from different locations.

To test it, login to the IMAP server via telnet like:

telnet HOSTNAME 143

Then look at what it sends for capabilities. In our example it was this:

OK [CAPABILITY +SPECIAL-USE STARTTLS LOGINDISABLED AUTH=CRAM-MD5] Dovecot ready.

Everything here looked necessary except one part: +SPECIAL-USE

So we grepped the contents of /etc/dovecot/conf.d/* for special and found the offending config and removed it.

Yours may not be exactly the same, but a similar tactic should resolve the problem.